[0001] The invention relates to a current mirror circuit for producing an output current
flowing in an output branch corresponding to a reference current flowing in an input
branch located between a supply voltage terminal and ground, including a first bipolar
transistor in the input branch through the collector/emitter circuit of which the
reference current furnished by a current source connected to the collector flows and
a second bipolar transistor in the output branch through the collector/emitter circuit
of which the output current flows, the bases of the two transistors being connected
to each other.
[0002] Current mirror circuits are used to generate a current in an output branch corresponding
as precisely as possible to a current flowing in an input branch. It is also possible
to generate the current in such a circuit in the output branch so that it is available
in a precise ratio to the current in the input branch. In a known circuit of this
kind, as shown in Fig. 2, two bipolar transistors 10, 12 are provided, the first of
which is located in the input branch whilst the second is located in the output branch.
The reference current I
r flowing in the input branch for mirroring in the output branch as the current I
a is generated by a current source 14. As evident, the bases of the two transistors
10 and 12 are connected to each other and the base current I
b required by these transistors is furnished via the drain/source circuit of a MOS
field-effect transistor 16 the gate of which is connected to the collector of the
transistor 10. Since the gate of the MOS field-effect transistor 16 absorbs no current
it is the reference current I
r generated by the current source 14 that flows through the collector/emitter circuit
of the transistor 10. This current then flows due to the known current mirror effect
also through the collector/emitter circuit of the transistor 12 so that the desired
effect is achieved, namely that in the output branch of the circuit a current I
a flows corresponding to the reference current I
r.
[0003] Although this known circuit as shown in Fig. 2 satisfies the cited condition very
precisely it has drawbacks when only a small supply voltage V
DD is available as is often the case in modern battery-powered electronic instruments.
As evident a voltage materializes at point A of the circuit corresponding to the sum
of the base/emitter voltage of the bipolar transistor 10 and the threshold voltage
V
th of the MOS field-effect transistor 16. These two voltages amount to roughly 0.7 V
so that at point A a voltage of at least 1.4 V occurs. When, however, the supply voltage
V
DD amounts only to 1.8 V as may very well be the case in practical applications then
a voltage of not more than 0.4 V is available for operation of the current source
14 which is not sufficient to configure the current source 14 with simple means to
reliably furnish the desired reference current I
r. Thus, the circuit as shown in Fig. 2 is only suitable for operation with higher
supply voltages.
[0004] The invention is based on the object of configuring a current mirror circuit of the
aforementioned kind so that it is able, despite low operating voltages, to mirror
the reference current flowing in the input branch with high accuracy in the output
branch.
[0005] This object is achieved in a current mirror circuit of the aforementioned kind by
a further current mirror circuit having an input branch located between the supply
voltage terminal and ground and an output branch located between the supply voltage
terminal and the connected bases of the two bipolar transistors for generating a base
current for these transistors, a current source controlled by the collector voltage
of the first bipolar transistor being located in the input branch of this further
current mirror circuit, the output current of this current source being mirrored in
the output branch of this further current mirror circuit.
[0006] In the current mirror circuit in accordance with the invention the base current required
for the bipolar transistors is produced by a further current mirror circuit capable
of producing this base current in its output branch without requiring current to be
branched from the reference current I
r. Due to making use of this further current mirror circuit and its current source
located in the input branch between the supply voltage terminal and ground the unavoidable
drop in voltage is greatly reduced so that the voltage remaining for operating the
current source furnishing the reference current I
r in the input branch becomes corresponding larger. The current mirror circuit is thus
able to mirror the reference current I
r in its output branch with high accuracy even when the supply voltage is low.
[0007] The invention will now be detailled by way of example with reference to the drawing
in which:
- Fig. 1
- is a circuit diagram of a current mirror circuit in accordance with the invention
and
- Fig. 2
- is a circuit diagram of a prior art current mirror circuit.
[0008] Referring now to Fig. 1 there is illustrated the current mirror circuit containing
as its basic components the two bipolar transistors 10 and 12 as well as the current
source 14 furnishing the reference current I
r. The output current I
a to be generated flows through a load resistor R.
[0009] The circuit as shown in Fig. 1 contains a further current mirror circuit consisting
of two p-channel MOS field-effect transistors 16 and 18 as well as an n-channel MOS
field-effect transistor 20 acting as the current source. The gates of the p-channel
MOS field-effect transistors 16 and 18 are connected to each other whilst their sources
receive the supply voltage V
DD. The drain of the p-channel MOS field-effect transistor 16 is connected to the gates
of these two MOS transistors. Furthermore, the drain of the p-channel MOS field-effect
transistor 16 is connected to the drain of the n-channel MOS field-effect transistor
20 whose source is directly grounded.
[0010] As evident from Fig. 1 a voltage occurs at the point A which corresponds to the threshold
voltage V
th of the n-channel MOS field-effect transistor 20. This means that for the same supply
voltage V
DD for operating the current source 14 a voltage is available which as compared to that
of the current mirror circuit as shown in Fig. 2 is higher by the base/emitter voltage
V
be. For a supply voltage V
DD of 1.8 V and a threshold voltage V
th of 0.7 V a voltage of 1.1 V is thus available at the circuit point A. It is with
this voltage that the current source 14 can be operated by simple ways and means.
The wanted object, namely to mirror the reference current I
r flowing in the input branch precisely in the output branch is thus achieved by very
simple circuitry.
1. A current mirror circuit for producing an output current flowing in an output branch
corresponding to a reference current flowing in an input branch located between a
supply voltage terminal and ground, including a first bipolar transistor in the input
branch through the collector/emitter circuit of which the reference current furnished
by a current source connected to the collector flows and a second bipolar transistor
in the output branch through the collector/emitter circuit of which the output current
flows, the bases of the two transistors being connected to each other, characterized by a further current mirror circuit (16, 18, 20) having an input branch located between
the supply voltage terminal and ground and an output branch located between the supply
voltage terminal and the connected bases of the two bipolar transistors (10, 12) for
generating a base current (Ib) for these transistors, a current source (20) controlled by the collector voltage
of the first bipolar transistor (10) being located in the input branch of this further
current mirror circuit (16, 18, 20), the output current (2Ib) of said current source (20) being mirrored in the output branch of said further
current mirror circuit (16, 18, 20).
2. The current mirror circuit as set forth in claim 1, characterized in that said controlled current source is an n-channel MOS field-effect transistor (20) whose
source is grounded, that said further current mirror circuit contains two p-channel
MOS field-effect transistors (16, 18) whose gates are connected to each other, the
drain of the one p-channel MOS field-effect transistor (16) being connected to the
drain of said n-channel MOS field-effect transistor (20) and its gate, and the drain
of the other p-channel MOS field-effect transistor (18) being connected to the bases
of said two bipolar transistors bipolar transistor (10, 12) whilst the sources of
said p-channel MOS field-effect transistors (16, 18) are connected to the supply voltage
terminal.
